Aug 15, 2019 Pageview：122
I. characteristics of lithium ion battery
Lithium is the smallest and most active metal on the periodic table. Small volume so high capacity density, widely welcomed by consumers and engineers. But too much chemical activity can be dangerous. When exposed to air, lithium can explode in a violent oxidation reaction with oxygen. To improve safety and voltage, scientists have developed materials such as graphite and lithium cobalt oxide to store lithium atoms. The molecular structure of these materials forms nano-sized, tiny storage cells that can be used to store lithium atoms. That way, even if the battery shell breaks and oxygen enters, the molecules are too big to fit into the tiny cells, preventing the lithium atoms from coming into contact with oxygen and exploding.
The principle of lithium ion battery enables people to obtain its high capacity density and achieve the purpose of safety at the same time. When a lithium-ion battery is charged, the positively charged lithium atoms lose electrons and oxidize to lithium ions. The lithium ions swim through the electrolyte to the negative electrode, where they enter a cell, where they gain an electron and are reduced to lithium atoms. Discharge, the whole process reversed. In order to prevent the battery positive and negative directly touch and short circuit, the battery will be added with a number of thin holes diaphragm paper, to prevent short circuit. Good diaphragm paper can also be in the battery temperature is too high, automatically close the hole, let the lithium ion can not pass through, in order to waste martial art, to prevent the danger.
Protective measures: lithium battery cells overcharge to a voltage higher than 4.2v, will start to produce side effects. The higher the overcharge voltage, the higher the risk. When the battery voltage is higher than 4.2v, the number of lithium atoms left in the anode material is less than half, and the cell often collapses, causing a permanent drop in battery capacity. If you keep charging, because the anode's cell is already full of lithium atoms, subsequent lithium metals will accumulate on the anode's surface. These lithium atoms grow dendritic crystals from the negative surface in the direction of the lithium ions. These lithium metal crystals will pass through the diaphragm paper, causing the positive and negative poles to short circuit. Sometimes the battery will explode before the short circuit occurs. This is because in the over-charging process, the electrolyte and other materials will crack and produce gas, making the battery shell or pressure valve bulge and crack, letting the oxygen go in and react with the lithium atoms accumulated on the negative electrode surface, and then explode.
Therefore, when charging a lithium battery, the upper limit of the voltage must be set, so that the battery life, capacity and safety can be considered at the same time. The optimal upper limit of charging voltage is 4.2v. When the lithium cell discharge must also have the voltage lower limit. When the cell voltage is below 2.4v, some of the material begins to break down. Since the battery will discharge itself, the longer the discharge, the lower the voltage will be. Therefore, it is better not to put 2.4v to stop the discharge. Between 3.0v and 2.4v, lithium batteries release only about 3% of their capacity. Therefore, 3.0v is an ideal cut-off voltage for discharge. In addition to voltage limitation, current limitation is also necessary during charging and discharging. When the current is too high, the lithium ions can accumulate on the surface of the material before they have time to enter the cell. When these ions gain electrons, they create crystals of lithium atoms on the material's surface, which, like overcharging, can be dangerous. In case the battery case breaks, it will explode.
Therefore, the protection of lithium-ion batteries should include at least three items: upper limit of charging voltage, lower limit of discharging voltage and upper limit of current. In general, lithium battery pack, in addition to lithium battery core, there will be a protective plate, this protective plate is mainly to provide these three protection. However, the protection of the three protection plate is obviously not enough, lithium battery explosion is still frequent worldwide.
To ensure the safety of the battery system, the cause of the battery explosion must be analyzed more carefully.
Ii. Cause of battery explosion
1. Large internal polarization;
2. The electrode sheet absorbs water and reacts with the electrolyte to form gas drum;
3. The quality and performance of electrolyte itself;
4. During the infusion, the infusion volume fails to meet the process requirements;
5. Poor sealing performance of laser welding in the installation and preparation process, air leakage.
6. Dust, the extremely thin dust is easy to cause micro-short circuit at first, the specific reason is unknown;
7. The anode and cathode plates are thicker than the process range, making it difficult to enter the shell;
8. The sealing problem of liquid injection and the poor sealing performance of steel ball lead to gas drum;
9. The incoming material of the shell is thick, and shell deformation affects the thickness;
Iii. Explosion type analysis
The type of cell explosion can be classified as external short circuit, internal short circuit and overcharge.
The outer part refers to the outer part of the cell, including the short circuit caused by poor insulation design inside the battery pack. When a short circuit occurs outside the cell and the electronic components fail to cut off the circuit, high heat will be generated inside the cell, causing part of the electrolyte to vaporize and stretch the battery housing. When the internal temperature of the battery is up to 135 degrees Celsius, the diaphragm paper of good quality will close the pore, the electrochemical reaction will be terminated or almost terminated, the current will drop sharply, and the temperature will also drop slowly, thus avoiding explosion. However, poor pore closure, or diaphragm paper with no pore closure at all, will cause the battery temperature to continue to rise, causing more electrolyte to vaporize, eventually bursting the battery housing and even raising the battery temperature enough to cause the material to burn and explode.
The internal short circuit is mainly caused by the burr of copper foil and aluminum foil piercing the diaphragm, or the dendritic crystal of lithium atoms piercing the diaphragm. These fine, needle-like metals can cause a micro-short circuit. Because the needle is very thin and has a certain resistance value, the current is not necessarily very large. The burr of copper and aluminum foil is caused in the production process, and the observable phenomenon is that the battery leakage is too fast, most of which can be screened out by the cell factory or assembly factory. And because the burrs are small, they can sometimes burn out, allowing the battery to return to normal. Therefore, the probability of explosion due to burr short circuit is not high. Such a statement can be from the cell factory inside are often charging soon, the voltage is low on the bad battery, but there are few explosions, statistical support. Therefore, the explosion caused by internal short circuit is mainly caused by overcharge. Because, after overcharging, you have acicular lithium crystals all over the plate, puncture points all over the place, micro-shorts all over the place. As a result, the battery temperature will gradually rise, and eventually the high temperature will release the electrolyte gas. In this case, whether the temperature is too high to cause the material to burn and explode, or the shell is first broken, so that the air goes in and the lithium metal is violently oxidized, is the end of the explosion.
However, the explosion caused by an internal short circuit caused by overcharging does not necessarily occur at the time of charging. It's possible that consumers stop charging their phones and take them out before the battery is hot enough to burn the material and produce enough gas to burst the battery's casing. At this time, the heat generated by numerous micro-short circuits will slowly increase the battery temperature, and after a period of time, the explosion will occur. The common description of consumers is that they pick up the phone and find it hot, throw it away and it explodes.
Based on the above explosion types, we can focus on the prevention of overcharge, the prevention of external short circuit, and improve the safety of the cell. Among them, overcharge prevention and external short circuit prevention belong to electronic protection, which is closely related to battery system design and battery pack installation. The emphasis of cell safety improvement is chemical and mechanical protection, which is closely related to cell manufacturers.
The page contains the contents of the machine translation.
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